In my opinion, gain is not the most important issue in choose the right step-up; the difference in gain of a 1:10 (20dB) to a 1:20 (26dB) step-up are really not that big. 6dB is easily compensated by the volume control. Also, it really depends on what gain your line stage has, some have 6dB, some 10dB some as high as 23dB. Gain is important but it is not the central issues in selecting the right step-up.
When selecting step-ups I would rather focus on the correct loading - The XV-1 has an internal impedance of 6 Ohm and Dynavector recommends 30 Ohm loading which is about right. A 1:20 step-up achieves a reflected loading of 117 Ohm. With a 22 KOhm additional resistor on the secondary (i.e. 15KOhm total loading) you get 37 Ohm. Certainly in the same transformer with more taps, I most often preferred the tap that gives the right loading without using extra resistors, irrespective of gain.
Finally different step-up are designed with different secondary load impedance in mind, loading a 1:10 at 10 KOhm may not necessarily ideal. Instead loading on the primary (i.e. before the step-up) may be preferred. To make a long story short, a 1:10 may work or not and so may a 1:20. The best result is in trying.
When selecting step-ups I would rather focus on the correct loading - The XV-1 has an internal impedance of 6 Ohm and Dynavector recommends 30 Ohm loading which is about right. A 1:20 step-up achieves a reflected loading of 117 Ohm. With a 22 KOhm additional resistor on the secondary (i.e. 15KOhm total loading) you get 37 Ohm. Certainly in the same transformer with more taps, I most often preferred the tap that gives the right loading without using extra resistors, irrespective of gain.
Finally different step-up are designed with different secondary load impedance in mind, loading a 1:10 at 10 KOhm may not necessarily ideal. Instead loading on the primary (i.e. before the step-up) may be preferred. To make a long story short, a 1:10 may work or not and so may a 1:20. The best result is in trying.